In U.S. Pat. No. 4,540,884 there is described a method of mass analyzing a sample by the use of a quadrupole ion trap. Basically, a wide range of ions of interest are created in or stored in an ion trap during an ionization step. In one method, the r.f. voltage applied to the ring electrode of the quadrupole ion trap is then increased and trapped ions of consecutively increasing specific mass-to-charge ratio (m/z) exit the ion trap. These ions are detected to provide an output signal indicative of the masses of stored ions.
In U.S. Pat. No. 5,420,425, there is described an ion trap mass spectrometer for analyzing ions, and more particularly a substantially quadrupole ion trap mass spectrometer with an enlarged ion occupied volume. Described therein are electrode geometries that enlarge the ion occupied volume. Improved ion sensitivities, detection limits and dynamic ranges are realized for the same charge density in these devices, because the increased ion occupied volume allows for the storage of a greater number of ions. The ion trap geometries described apply to all modes of operation of substantially quadrupole ion traps, such as the mass selective instability mode, resonance excitation/ejection, and MS.sup.n.
In U.S. Pat. No. Re 34,000 there is disclosed a method of performing MS/MS in a quadrupole ion trap. Ions stored within the quadrupole ion trap are excited by applying an excitation voltage of predetermined frequency for a predetermined time across the end caps of the ion trap. Ions that follow orbital trajectories at a frequency resonant or near resonant with the excitation frequency gain kinetic energy as they absorb AC power. The ions involved in this excitation undergo dissociation by ion molecule or ion/ion collisions within the trap (collision-induced dissociation). The dissociated ions are then caused to leave the ion trap by changing the trapping voltages as described above to obtain a mass spectrum of the dissociated ions.
The resonance excitation (RE) method has been found to be very effective in fragmenting ions in a quadrupole ion trap and is very efficient in terms of converting parent ions into product ions without much loss of total charge. However, in order to obtain optimal fragmentation efficiency for a particular ion, the amplitude of the applied resonance excitation voltage must often be tuned for each ion of interest. It has been argued that fragile ions, for example a 2+ or 3+ multiply charged ion should in general be more easily fragmented than the 1+ ion of the same mass, and therefore would require less resonance excitation voltage amplitude. Charge state and other structural characteristics were often thought to be the primary cause of the variations in required excitation voltage amplitude. The fact that different ions require different excitation voltage amplitudes precludes the ability of doing automated experiments where the choice of parent ion is not predetermined but made in real time in a chromatographic or other fast time scale. Under these circumstances, tuning of the voltage amplitude is not practical, since in general it is a time-consuming process.
In addition to this limitation, the particular setting of resonance excitation voltage amplitude required to fragment a given ion optimally can differ from one instrument to another. These differences depend on variations in instrumental parameters such as power supplies and other electronics, as well as variation in helium and background gas pressures. Consequently, the same excitation voltage amplitude used on multiple instruments may not give identical results.
Both of these limitations can be significantly improved upon by using the present invention which attempts to normalize out the primary variations in optimal resonance excitation voltage amplitude for differing ions, and also the variations due to instrumental differences.